1. Field of the Invention
The present invention relates to a focus adjustment apparatus, a camera system, and a focus adjustment method to perform focus adjustment by a phase difference AF method.
2. Description of the Related Art
Conventionally, a phase difference AF method has been used for focus adjustment in a camera system and the like. This phase difference AF method obtains a two-image interval value (also called image phase difference information), and converts this two-image interval value into a defocus amount using a conversion coefficient (also called AF sensitivity). This conversion coefficient varies because of individual variation in an imaging element, and individual variation in a photographic lens, in the case of a so-called image plane phase difference AF method. For the individual variation in the imaging element, the influence of variation in the manufacturing quality of an on-chip micro-lens for pupil division is large. Further, for the individual variation in the photographic lens, the influence of variation in the manufacturing quality of the opening diameter of a diaphragm is large. An imaging apparatus to correct such a conversion coefficient is described in Japanese Patent Laid-Open No. 2012-220925 (hereafter referred to as “Patent Literature 1”). This Patent Literature 1 discloses that, when the two-image interval value is obtained from image data which is calculated by the addition of pixel data sets obtained from a focus detection pixel a plurality of times with lapse of time and is converted into a defocus amount, the conversion coefficient is corrected and the defocus amount is calculated in order to cope with the optical change of the photographic lens during this lapse of time.
Above Patent Literature 1 discloses that the conversion coefficient relating to the temporal change of an optical system is corrected. However, it is not disclosed that the variation of the conversion coefficient caused by the individual variation in the imaging element or an interchangeable lens is corrected. In the correction of the variation in the conversion coefficient, although adjustment can be performed for absorbing the influence of the individual variation in the imaging element, it is very difficult to adjust variation corresponding to the individual variation in the interchangeable lens. Further, it is also difficult to adjust a variation component of mutual influence between the individual variation in the imaging element and the individual variation in the interchangeable lens.
For example, FIG. 2 shows the two-image interval by the horizontal axis and the defocus amount by the vertical axis, and L1 to L3 are conversion coefficient curves which are assumed to vary caused by the individual variation in the imaging element or the interchangeable lens. Here, when it is assumed that L3 shows a true conversion coefficient curve in the case that the two-image interval value is X and L1 shows a currently used conversion coefficient curve, the defocus amount becomes y and a value smaller than a true defocus amount y′ is calculated. As a result, lens drive has an insufficient state, and it is necessary to detect the two-image interval value and perform the lens drive (also called LD) again.
Accordingly, the number of lens drive times becomes large and AF time becomes long. Further, when the AF sensitivity is shifted, it is not possible to accurately detect the defocus amount during continuous AF or continuous shooting AF, and therefore the moving object predicting performance becomes degraded.